Liquid crystal materials (LCs) consist of rod-like molecules that prefer to align parallel to each other and are capable of being aligned by applied electric fields. The long-range order caused by the local alignment of molecules enables the “director”, the average molecular direction, to be defined. The long and short axes of these molecules exhibit different optical properties and certain molecular orientations can therefore alter the polarisation and intensity of incident light. These reorientation and polarisation effects can be utilised in conjunction with optical elements such as polarising and reflective layers to produce optical devices such as optical switches, changeable phase gratings or displays.
Existing LC devices commonly have two transparent substrates, with semi-transparent electrodes that may be patterned into pixels on the inner side of each substrate, sandwiching a liquid crystal (LC) material between them. A reflective coating may be placed at one substrate if the device is to be used to reflect incident light once it has passed through the liquid crystal layer rather than in a transmissive mode. Optical polarisers are usually placed on the outer surface of one or both substrates. Between the liquid crystal layer and each electrode/substrate an alignment layer is used to specify the orientation of the LC molecules close to the substrate. Each pixel may be addressed “passively” using a voltage applied across the pixel row and column electrodes or “actively” using thin film transistors to selectively apply an electric field across a single pixel. This electric field may be used to switch the liquid crystal molecules between two orientational states, each with a different effect on the light passing through the LC layer such that, depending on the state, the light may either be transmitted through or blocked by the polariser(s).
One method of achieving this is through the use of twisted nematic configurations. In these devices, alignment layers are used to orient the LC molecules close to opposing substrates in perpendicular directions to each other. In the state where no electric field is applied across a pixel, the perpendicular LC alignment close to the substrates results in a 90° twist in LC orientation as one moves from surface to surface. This has the effect of rotating the polarisation of the light through 90°. When polarisers are placed parallel to the alignment directions at each substrate, incident light becomes initially polarised by the first polariser in one direction, then the polarisation direction of the light is rotated 90° by the LC to coincide with the second polariser at the other substrate, such that light is output from the pixel. Coloured filters may be placed over the pixel to produce coloured pixels. When a voltage is applied between the electrodes of a pixel, the LC aligns with the direction of the field, i.e. perpendicular to the plane of both substrates. In this state, as the LC molecules are aligned parallel to the direction of the propagation of the light, no change in polarisation of the light due to the LC configuration takes place. Thus, polarised light from the first polariser is blocked by the second polariser in the perpendicular direction. This type of device is “transmissive” since the incident light passes through the device from a light source (i.e. a backlight in a display module) on one side to the observer on the other.
An alternative method of displaying information using a liquid crystal device is through the use of a “reflective” LC device. A typical reflective device is constructed in a similar way to a transmissive device except the polariser furthest away from the observer is replaced by a reflective element. Light is incident from the same side of the device as the observer and is initially polarised by the polariser in one direction. One of the possible LC configurations is such that the polarisation direction of the light is rotated by 45° by the LC, before it is reflected by the reflective element and then rotated again by 45° by the LC. The total rotation of the polarisation of light by 90° means that the light is blocked by the polariser. When a voltage is applied between the electrodes of a pixel, a reorientation of the LC occurs such that the LC configuration does not affect the polarisation of light and the incident light is polarised, transmitted through the LC, reflected and transmitted through the LC again with no change in polarisation direction, so that the polariser allows the light to be emitted and observed. Coloured filters may be placed over the pixel to produce coloured pixels.
LC displays have many advantages such as being very flat, light and robust when compared with other display types such as cathode ray tubes. As such they are ideal for small portable devices such as mobile phones and PDAs. However, they have a high-energy demand due to the need for constant power to be applied to hold a pixel in one state. Furthermore, transmissive displays typically require backlighting from a light source to achieve a bright picture with high contrast, which in turn further increases the power consumption, leading to a shortened battery life. In addition, since these systems employ out of plane switching, where the LC molecules align themselves perpendicularly to the plane of the substrates in the presence of the electric field, birefringence effects can lead to a loss of contrast when viewed from the side. The resultant geometry may also lead to colour distortion due to parallax effects.
Some of the problems associated with the devices described above are addressed by multistable or, specifically, reflective bistable liquid crystal technology. In a multistable liquid crystal device, the LC has more than one stable director configuration. As such, once switched into a stable state, the LC remains in that state until an electric field is applied to change the configuration. This type of operation requires less power since power is only supplied to change states and not supplied continuously to maintain a state. The reflective aspect of such a technology removes the need for a backlight, further reducing the power consumption.
In Thurston et al. IEEE Transactions on Electronic Devices Ed-27, no. 11, pp. 2069-2080 1980, LC devices having a number of possible multistable systems are described. In particular, zenithal multistable devices are described which produce distortion out of the substrate plane. In-plane multistable devices are also described that rely on patterned alignment regions on the substrates.
U.S. Pat. No. 4,333,708 describes an example of a bistable LC device having a number of multistable modes that involve the motion of singular points or “disclinations” lying parallel to the device substrates. EP 0,517,715 and WO92/00546 describe other examples of bistable LC devices, in which surface treatments (evaporation of SiO) are used to produce a bistable surface alignment layer. This surface exhibits two possible director configurations. By switching the director at the surface with the application of a suitable voltage waveform, switching between the two states is possible.
Another possible bistable device is described in U.S. Pat. No. 5,796,459. This has substrate surfaces that have been treated so that bigratings exist on one or both substrates. This bigrating arrangement creates two different possible angular directions in which the LC molecules can lie. WO 97/14990 describes a further example of a bistable device. This has a surface alignment monograting on at least one of the substrates. The surface monograting has a groove height to width ratio that leads to approximately equal energy for two director alignment arrangements. The director alignment arrangements differ from each other primarily by angle of the director from the plane of the substrate. The device is switched using appropriate voltage pulses.
Yet another approach to producing bistable liquid crystal devices involves having an array of either posts or holes placed on one of the substrates. Devices of this type are described in EP 1,271,225 and EP 1,139,151. The presence of posts or holes allows multiple director orientations to be stable. The difference between the director orientations is primarily in the difference in director angle from the main plane of the substrates. Switching between these states is achieved using appropriate voltage pulses coupling to the molecular dipoles.
In all of the above devices, each stable state has a different effect on the polarisation of light and this can be used in conjunction with suitably oriented polarisers to allow or block the transmittance of light. These systems may be used with light transmitted through the device or reflected from a surface at the back of the device. Variations in surface treatments on the sub-pixel level may be used to achieve greyscale. Whilst these provide some advantages over more conventional arrangements, they suffer from the fact that the switching is out of plane, which can lead to loss of contrast at oblique viewing angles and colour deformations due to parallax errors.
Some of the devices described above may contain LC defect regions. When LCs are enclosed within a container, the molecular direction is influenced by the container surfaces. This can lead to conflicts at certain regions, resulting in defects. In these defect regions, the molecules align themselves in such a fashion so that a high distortion energy structure, associated with a reduction in molecular order, is formed, as discussed by Repnik et al. in European Journal of Physics, Vol. 24, pages 481-492 (2003). Depending on factors such as the dimensions and shape of the container, topography of the surface, temperature, applied electric field and the surface energy of the walls, several configurations of the liquid crystal molecules may be possible. Each configuration may have varying defect positions and alignment of bulk liquid crystal molecules. Defects are, however, generally regarded as undesirable as they lower device efficiency. Hence, in most known devices steps are taken to remove such defects or avoid their formation altogether.